Last week I visited New York City, and while I was there I was invited to the rather excellent Mysterious Bookshop in TriBeCa. There, I signed a big box full of The Lazarus Gate. As far as I know, it's the only place in the US presently where you can get signed copies, so do drop them a line if you want one!
If you're in the city, do visit The Mysterious Bookshop. It's truly one of the best indie bookstores around, and has the enviable position of housing one of the largest selections of Sherlock Holmes books and periodicals in America!